Method of manufacturing a precisely aligned microlens array

ABSTRACT

A method of manufacturing a microlens array requires at least two fiducial marks formed on a surface of a transparent medium opposite the microlens array. Additional optical features formed on the transparent medium adjacent the microlens array enables precise locationing of fiducial marks on an opposing surface when such surface is exposed to a collimated beam of light. The location of fiducial marks using the method of the invention is about 1 micron or less.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is related to U.S. application Ser. No.(Docket 83858), filed - - - , by Border, et al., and entitled, “MethodOf Forming Fiducial Marks On A Micro-Sized Article;” U.S. applicationSer. No. (Docket 83859), filed - - - , by Border, et al., and entitled,“Microlens Array;” U.S. application Ser. No. (Docket 83861), filed - - -, by Border, et al., and entitled, “Double-Sided Microlens Array AndMethod Of Manufacturing Same;” U.S. application Ser. No. (Docket 83862),filed - - - , by Border, et al., and entitled, “Laser Array And MethodOf Making Same;” and, U.S. application Ser. No. (Docket 83863),filed - - - , by Border, et al., and entitled, “Fiber Optic Array AndMethod Of Making Same.”

FIELD OF THE INVENTION

[0002] The invention relates generally to the field of microlens lensarrays. More particularly, the invention concerns forming fiducial markson optical articles that require precise alignment in an optical systemcontaining the microlens array.

BACKGROUND OF THE INVENTION

[0003] Optical systems, such as imaging systems, telecommunicationsdevices, micro-optical systems, micro-mechanical systems, etc., aretypically constructed of several different lenses and optical articlesto deliver the desired optical performance. To avoid large overalllosses in the optical system, the alignment of each lens and opticalarticle with subsequent lenses and optical articles must be veryprecise. Fiducial marks are often created on the lenses and opticalarticles outside the optical area to serve as a reference point duringalignment. Fiducial marks are particularly important in the case ofaspheric lenses and lens arrays where it is difficult to identify thecenter of the lens during alignment activities. Fiducial marks are alsovery important for fiber optic arrays and laser arrays where multiplefeatures dictate the need for a shared alignment reference which islocated precisely in relation to all the optical features. As opticalsystems get smaller for fiber optics applications, liketelecommunications and optical sensors, the need increases for precisealignment of the optical components and the accuracy of the associatedfiducial marks. Alignment specifications of two (2) microns are nowcommon with a desire to deliver submicron alignment accuracy.Consequently, the fiducial marks must be located with an accuracy of 1micron or better.

[0004] Fiducial marks are well known in the semi-conductor manufacturingindustry as an important tool for making multilayer semiconductors. Inthis case, the fiducial marks are incorporated as part of thesemiconductor circuit plan. Due to the thinness (50-100 micron) of thesemiconductor layers used in making multilayer semiconductors, thefiducial marks of multiple semiconductor layers can be viewedsimultaneously using a high magnification microscope. The highmagnification microscope aids in positioning the fiducial marks of onesemiconductor layer over the fiducial marks of another semiconductorlayer during the alignment process.

[0005] Forming fiducial marks in optical articles raises specialchallenges in that optical surfaces are typically relatively thick,often well over a 1000 micron in thickness. This is the case even in amicrolens array that has microlenses that are well under a millimeter indiameter. The thickness of the microlens array makes it virtuallyimpossible to accurately locate a fiducial mark by looking through themicrolens array due to optical limitations. On the one hand, thelocation accuracy of the fiducial mark relative to the optical articleis limited because the fiducial mark is displaced by refracted lightpassing through the microlens array material. Moreover, the thickness ofthe microlens array limits how close the microscope used for identifyingthe microlens array can be positioned to the fiducial mark.Consequently, only lower magnification microscopes can be used to lookat the fiducial. Therefore, for optical articles, a method of applying avery accurately located fiducial mark on the side opposite to theoptical article is needed.

[0006] In U.S. Pat. No. 6,005,294, by Tsuji et al., Dec. 21, 1999,entitled “Method Of Arranging Alignment Marks,” a method of makingsemiconductor devices uses multiple fiducial marks in such a way thatthe area occupied by the fiducial marks is reduced and the manufacturingproductivity is correspondingly increased. While this patent doesdescribe the state of the art for making semiconductor devices, thealignment process described therein is not appropriate for opticalarticles like lens arrays. As mentioned, in lens arrays, the significantthickness of the various lenses makes it impossible to view fiducialmarks from multiple optical articles simultaneously due to theseparation distance imparted by the material thickness of the lenses.

[0007] Also, U.S. Pat. No. 5,850,276, by Ochi et al., Dec. 15, 1998,entitled “Method Of Making LCD Device Having Alignment Mark Made Of SameMaterial And Formed At Same Time As Microlenses” and U.S. Pat. No.5,771,085, by Ochi et al., Jun. 23, 1998, entitled “LCD Device With anAlignment Mark Having Same Material As Microlenses” each describe aprocess for molding fiducial marks into a microlens screen used forliquid crystal display devices. In these patents the shapes of thefiducial marks are also described in detail. The fiducial marks asdescribed are protrusions in the shape of a cross or several othervariations, located on the same side as the microlenses. The protrusionscan be semicircular in cross section or another shape as long as thegrooves between the protrusions stand out as dark lines when viewed witha reflecting microscope. The references recognize that lenscharacteristics, such as thickness, interfere with the ability toidentify underlying fiducial marks. Further, the references show someappreciation for useful geometries of fiducial marks and for fiducialmarks molded along with a microlens array. However, neither of thepatents show appreciation for fiducial marks applied on the sideopposite the optical surfaces in the microlens array. Furthermore, thereis no appreciation by either of the references that advantages can begained with a molded fiducial mark having lens characteristics.

[0008] Moreover, U.S. Pat. No. 6,096,155, by Harden et al., Aug. 1,2000, entitled “Method Of Dicing Wafer Level Integrated Multiple OpticalElements” discloses the use of fiducials to aid in alignment ofmicrolenses on wafers during the bonding of multiple wafers togetherprior to dicing. This patent generally teaches making integratedmultiple optical elements with features to help control the thickness ofadhesives and solders used to bond together the wafers. While effectiveuse of the fiducial marks is described, there is absolutely no mentionof ways to improve alignment of fiducial marks on one side with theoptical element on the other side of the wafer. The techniques ofembossing and molding fiducial marks, described in the patent, bothsuffer from locational inaccuracies from one side to the other o theorder of plus or minus ten (10) microns. In molded microlenses andmicrolens arrays this inaccuracy is not acceptable.

[0009] Furthermore, U.S. Pat. No. 4,598,039, by Fischer et al., Jul. 1,1986, entitled “Formation Of Features In Optical Material” describes theuse of a laser to remove optical material in a controlled fashion. Thelaser can be used directly on the optical material or a layer ofablative absorber material can be put onto the surface of the opticalmaterial to enhance the coupling to the laser. This ablative techniqueis well suited to making fiducial type marks for alignment. However, thereference does not show appreciation for how to align the laser with alens array that is located on the opposite side from the desiredlocation for the fiducial marks.

[0010] Therefore, a need persists in the art for a method of formingfiducial marks onto optical articles and optical arrays on a surfaceopposite the optical article surface that enables precise alignment ofthe articles and optical arrays. Moreover, there is a compelling needfor a special optical feature molded along with optical surfaces tofocus light onto an opposing surface of the optical article or opticalarray thus enabling the formation of a fiducial mark onto the opposingsurface with great accuracy.

SUMMARY OF THE INVENTION

[0011] It is, therefore, an object of the invention to provide a methodfor making fiducial marks on optical articles where the fiducial mark islocated on a surface opposite the surface of the optical article.

[0012] It is a further object of the invention to utilize an opticalfeature made in conjunction with the optical article that focuses a highintensity beam of light onto the surface opposite the surface of opticalarticle to thereby form a fiducial mark.

[0013] To accomplish these and other objects, features and advantages ofthe invention, there is provided, in one aspect of the invention, amethod of manufacturing a microlens array having at least two fiducialmarks arranged on a surface opposite the microlens array includesproviding a transparent medium for mountably supporting the microlensarray. In this embodiment, the transparent medium has a first surfacefor supporting the microlens array and a second surface opposite thefirst surface for receiving fiducial marks. A microlens array is formedon the first surface of the transparent medium and first and secondoptical features are formed on the first surface of the transparentmedium adjacent to the microlens array. So as to distinguish thefiducial marks, at least a portion of the second surface of thetransparent medium is altered before forming the fiducial marks thereon.In this embodiment of the invention, at least two fiducial marks arethen formed on the altered portion of the second surface correspondingprecisely to each one of the first and second optical features.

[0014] Consequently, the present invention has numerous advantages overprior art developments, including: it results in precision locating offiducial marks; it is a far superior method of aligning optical articlesin an array; and, it is significantly easier to implement since allrequired optical features are formed with the same forming process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing as well as other objects, features and advantagesof this invention will become more apparent from the appended Figures,wherein like reference numerals denote like elements, and wherein:

[0016]FIG. 1a is a perspective view of a prior art lens array withfiducial marks located on the side opposite the lens surfaces;

[0017]FIG. 1b is a perspective view of an optical system made inaccordance with the method of the invention;

[0018]FIG. 2 is an elevated side view of a microlens array set in atransparent medium having optical features formed in accordance with themethod of the invention;

[0019]FIG. 3 is an elevated side view of a microlens array set in atransparent medium with a fiducial mark forming means arranged forforming fiducial marks on an opposing surface of the transparent medium;

[0020]FIG. 4 is a perspective view of the optical article havinggenerally circular fiducial marks on a surface opposite the opticalarticle;

[0021]FIG. 5 is a perspective view of the optical article of theinvention having a generally linear crossed fiducial mark on a surfaceopposite the optical article;

[0022]FIGS. 6a and 6 b are perspective views of an alternativeembodiment of the invention having a plurality of optical articles oneither face of the transparent medium with corresponding fiducial markson opposing surfaces in the transparent medium opposite the opticalarticle;

[0023]FIG. 7 is a perspective view of an alternative embodiment of theinvention comprising a laser array; and,

[0024]FIG. 8 is a perspective view of another embodiment of theinvention comprising a fiber optic array.

DETAILED DESCRIPTION OF THE INVENTION

[0025] A typical prior art microlens array 2 is illustrated in FIG. 1afor comparative purposes. According to FIG. 1a, microlens array 2 hasmultiple microlenses 1 mounted coincidentally on a mounting flange 3.Fiducial marks 7 are located on a surface 5 of mounting flange 3opposite the surface 6 of microlenses 1. Fiducial marks 7 would eitherbe directly molded onto surface 5 or would be applied after referencingan edge of the optical surface from the opposite side of mounting flange3. In the case of direct molding of the fiducial marks 7, moldmisalignment due to clearance in the alignment pins across the moldedparting line would limit the accuracy of fiducial mark 7 toapproximately 15 microns or more. Using the edge referencing technique,experience has taught that each measurement introduces approximately 2-5microns of inaccuracy. Since a minimum of three (3) measurements arerequired to identify an edge of a round lens, the total inaccuracy is aminimum of 6-15 microns to place the fiducial mark 7. While thisinaccuracy is usually acceptable for large optical articles, as the sizeof optics for applications such as fiber optics shrinks below 1000micron, the alignment accuracy required shrinks as well. Consequently,it is not uncommon for alignment accuracy of microlenses to be 5 micronsor better with some applications calling for 2 micron alignment.Obviously, the accuracy of the fiducial marks 7 must be better than thealignment accuracy required.

[0026] Turning now to FIG. 1b, fiducial marks 13 formed in an opticalarticle array, such as refractive lens array 11, using the method of theinvention is illustrated. In this embodiment, fiducial marks 13 are usedto align an optical assemblage 8 comprising refractive lens array 11 andlaser array 9. According to FIG. 1b, fiducial marks 13 on the lens array11 are precisely located on opposing surface 11 b of lens array 11. Toensure precise alignment of optical assemblage 8, each one of aplurality of precision through-holes 15 formed in laser array 9 isalignably centered over a corresponding fiducial mark 13 in lens array11. This process aligns each of the lasers 9 a in the laser array 9 witha refractive lens 1 a in the refractive lens array 11. After the opticalassemblage 8 is aligned, it is rigidly affixed typically by potting in asuitable adhesive material. Precise alignment of precision through-holes15 over the fiducial marks 13 is accomplished with a high powermicroscope (not shown) often with a computerized vision system linked toa computerized positioning system to automate the process.

[0027] Referring to FIGS. 2 and 3, an optical array 10 having accuratelylocated fiducial marks 24, 28 formed on an opposing surface 30 of atransparent substrate 12 is illustrated. According to FIGS. 2 and 3,optical articles, such as microlens array 22, 32, are supported onmounting surface 14 of transparent substrate 12 that is opposite surface30. Important to the invention, an additional optical feature 20(described below) is formed adjacent to the microlens array 22, 32 toaid in precisely forming fiducial marks at focal points 24, 28.According to FIG. 2, focal point 24 (corresponding to a fiducial mark)is then produced with a high intensity collimated beam of light 26. Asshown in FIG. 3, a laser source 27 may be used to produce such highintensity light 26. The additional optical feature 20 receives thecollimated beam of light 26 from laser source 27 and precisely focusesit onto opposing surface 30 of the microlens array 10. It is alsoimportant to the invention that prior to forming the fiducial marks 13at focal points 24, 28, surface 30 of the transparent substrate 12 isaltered or treated in the area where the fiducial marks 13 are to beformed. The objective of altering or treating surface 30 is to makesuitably visible fiducial marks 13 when exposed to the focused highintensity light 26. Suitable surface altering techniques include dipcoating, roughening, spin coating, vacuum coating, metallizing, amongothers.

[0028] Skilled artisans will appreciate that there are several processesthat may be used for forming a mold for making optical articles, such asoptical array 10, which includes additional optical feature 20 asdescribed. Such processes include lithographic printing, ink jetprinting, indentation, diamond turning and diamond milling, each ofwhich can deliver a position to position accuracy of 0.25 micron.Importantly, the method of the present invention uniquely uses theprocess for forming the microlens array 32 for also forming theadditional optical features 20 that precisely locates the fiducial marksat focal points 24, 28.

[0029] Referring to FIGS. 4 and 5, optical features having a variety ofconfigurations with refractive or diffractive lenses can be used tocreate various shaped fiducial marks. According to FIG. 4, a lens array40 has a plurality of lenses 41 formed in first surface 46 oftransparent medium 44. Generally round refractive lens feature 45 can beused to make a generally round fiducial mark 42 in second surface 48 oftransparent medium 44, opposite first surface 46 of the transparentmedium 44. Moreover, to produce a generally linear fiducial mark, agenerally linear lens feature is required (not shown). According to FIG.5, a generally crossed linear refractive lens feature 50 is used toproduce a generally crossed-shaped (X-shaped) fiducial mark 52. Thoseskilled in the art will now appreciate that other patterns for theoptical feature can be produced by a combination of refractive anddiffractive optical features.

[0030] Referring to FIGS. 6a and 6 b, in another embodiment of theinvention, double-sided optical arrays 58, 59 are illustrated. Accordingto FIG. 6a, double-sided optical array 58 has an arrangement of opticalarticles 60, 62 on either of opposing surfaces 61 a, 61 b in transparentmedium 61. Fiducial marks 69, 66 are formed on both opposing surfaces 61a, 61 b, respectively, by repeating the fiducial marking processdescribed hereinabove. According to FIG. 6b, alternatively, double-sidedoptical array 59 has optical features 72, 80 mounted on opposingsurfaces 70 a, 70 b of transparent medium 70. In this embodiment, twosets of fiducial marks 78, 83 are formed only on surface 70 b oppositesurface 70 a so the misalignment between the two optical articles 72, 80could be easily determined.

[0031] Referring again to FIG. 6a, double-sided optical array 58, moreparticularly, has a first plurality of lenses 60 matched to a secondplurality of lenses 62, both being mounted in opposing surfaces 61 a, 61b of transparent medium 61. Two complimentary sets of additional opticalfeatures 65, 68 are formed in either of opposing surfaces 61 a, 61 b,respectively. Optical features 65, 68 are used to form fiducial marks66, 69 on the opposing surfaces 61 b, 61 a, respectively. As shown inFIG. 6a, optical feature 65 has a generally round shape which forms agenerally round shaped fiducial mark 66 on the opposing surface 61 b. Inthe same alternative, double-sided optical array 58, a generally ringshaped optical feature 68 formed on surface 61 b produces a generallyring shaped fiducial mark 69. Alternatively, fiducial marks 66, 69 andoptical features 68, 65 can be used as matching reference marks tomeasure the relative alignment of the optical articles 60, 62 onsurfaces 61 a, 61 b by measuring the relative centering of the fiducialmarks 66, 69 from the optical features 65, 68.

[0032] It is the experience of the inventors that by using bothrefractive and diffractive lenses in the additional optical lensfeatures, a wide variety of fiducial mark shapes can be created to fitdifferent requirements. The additional optical lens feature can also bedesigned for different wavelengths if the fiducial marking is to be doneusing a light source that operates at a different wavelength than usedby the optical array.

[0033] Referring again to FIG. 6b, another embodiment of a double-sidedoptical array 59 is illustrated. As described above, a first pluralityof lenses 72 in optical array 59 has additional generally round opticalfeatures 74 formed on surface 70 a of transparent substrate 70. Opticalfeatures 74 provide precise focusing of the collimated beam of light(FIG. 2) onto opposing surface 70 b which forms a generally roundfiducial mark 78 on a treated portion 76 of opposing surface 70 b. Inthis embodiment, the second plurality of lenses 80 is formed on opposingsurface 70 b of transparent medium 70. Further, generally squarefiducial marks 83 surround generally round fiducial marks 78 that havebeen produced by optical features 74 formed on opposing surface 70 a.The alignment of the first plurality of lenses 72 to the secondplurality of lenses 80 is preferably determined by measuring themagnitude and direction of the de-centering, i.e., the distance from animaginary centerline passing through the lenses to the fiducial mark.

[0034] In FIGS. 7 and 8, two additional embodiments of the invention areillustrated. According to FIG. 7, a laser array 110, having lasers 90,includes two additional optical features or cross linear lens 92 thatproduce fiducial marks 94 in the form of a cross (X) on an opposingsurface 96 b. Lasers 90 may be arranged in openings in transparentmedium 96 or they may be bonded to first surface 96 a of transparentmedium 96. According to FIG. 8, a fiber optic array 120, having fiberoptic units 100 formed in transparent substrate 106, includes additionaloptical features 102 adjacent to fiber optic units 100 that are used toproduce fiducial marks 104 on an opposing surface 106 b of the fiberoptic array 120. The fiber optic units 100 may be formed in transparentsubstrate 106 or they may be bonded to first surface 106 a. The sameprocess, described above, for forming fiducial marks 94, 104, is used inthe present embodiments of the invention.

[0035] The invention has been described with reference to variousembodiments thereof. However, it will be appreciated that variations andmodifications can be effected by a person of ordinary skill in the artwithout departing from the scope of the invention.

PARTS LIST

[0036]  1 microlens  2 prior art microlens array  3 mounting flange  5surface of mounting flange 3  6 surface of mounting flange 3 supportingmicrolens 1  7 fiducial marks on opposing surface 5  8 opticalassemblage  9 laser array  9a lasers in laser array 9  10 optical array 11 refractive lens array  11a refractive lens in refractive lens array11  12 transparent substrate  13 fiducial marks for refractive lensarray 11  14 mounting surface  15 precision through-holes  20 additionaloptical feature  22 microlens array  24 focal point on an oppositesurface to the microlens array  26 high intensity collimated beam oflight  27 laser source  28 focal point produced by the collimated light26 passing through the additional optical features 20  30 fiducialmarking area on the opposite side of the microlens array 22  32 multiplelens refractive lens array  40 lens array  41 plurality of lenses  42generally round fiducial mark  44 transparent medium  45 generally roundrefractive lens feature  46 first surface of transparent medium 44  48second surface of transparent medium 44  50 crossed linear refractivelens feature  52 cross-shaped fiducial mark  58 alternative double-sidedoptical array  59 alternative double-sided optical array  60 opticalarticles (first plurality of lenses in optical array 58)  61 transparentmedium  61a, b opposing surfaces in transparent medium 61  62 opticalarticles (second plurality of lenses in optical array 58)  65 roundupper additional optical feature  66 found fiducial mark  68 ring-shapedlower additional optical feature  69 ring-shaped fiducial mark  70transparent medium  70a, b opposing surfaces in transparent medium 70 72 optical features (first plurality of lenses in lens array 59)  74round additional optical feature  76 treated portion of opposing surface70b  78 round spot fiducial marks  80 optical features (second pluralityof lenses in lens array 59)  83 square fiducial mark  90 lasers  92crossed linear lens on laser array 110  94 X-shaped fiducial marks onlower surface  96 transparent medium of laser array 110  96a firstsurface of transparent medium 96  96b second surface of transparentmedium 96 100 fiber optic units 102 additional optical feature 104X-shaped fiducial marks on lower surface 106 transparent medium of fiberoptic array 120 106a first surface of transparent medium 106 106bopposing surface of transparent medium 106 110 laser array 120 fiberoptic array

What is claimed is:
 1. Method of manufacturing a microlens array havingat least two fiducial marks arranged on a surface opposite saidmicrolens array, said method comprising the steps of: providing atransparent medium for mountably supporting said microlens array, saidtransparent medium having a first surface for supporting said microlensarray and a second surface opposite said first surface for receivingsaid at least two fiducial marks; forming said microlens array on saidfirst surface of said transparent medium and forming first and secondoptical features on said first surface of said transparent mediumadjacent to said microlens array; altering at least a portion of saidsecond surface of said transparent medium; and, forming said at leasttwo fiducial marks on said at least a portion of said second surfacecorresponding precisely to each one of said first and second opticalfeatures.
 2. The method recited in claim 1 wherein said step of formingsaid at least two fiducial marks includes the step of directing andfocusing a collimated beam of light through said first and secondoptical features onto said second surface thereby forming said at leasttwo fiducial marks.
 3. The method recited in claim 2 wherein saidcollimated beam of light is produced by a laser.
 4. The method recitedin claim 1 wherein said first and second optical features each has apredetermined configuration.
 5. The method recited in claim 4 whereinsaid predetermined configuration of said first and second opticalfeatures is generally circular thereby forming at least two generallycircular fiducial marks.
 6. The method recited in claim 4 wherein saidpredetermined configuration of said first and second optical features isgenerally linear thereby forming at least two generally linear fiducialmarks.
 7. The method recited in claim 4 wherein said predeterminedconfiguration of said first and second optical features is generallycrossed linear thereby forming at least two generally crossed linearfiducial marks.
 8. The method recited in claim 4 wherein saidpredetermined configuration of said first and second optical features isgenerally a diffractive element thereby producing at least two generallycomplex-shaped fiducial marks.
 9. The method recited in claim 1 whereinsaid step of altering includes the step of painting said at least aportion of said second surface of said transparent medium so as todistinguish said at least two fiducial marks formed thereon.
 10. Themethod recited in claim 1 wherein said step of altering includes thestep of metallizing said at least a portion of said second surface ofsaid transparent medium so as to distinguish said at least two fiducialmarks formed thereon.
 11. The method recited in claim 1 wherein saidstep of altering includes the step of vacuum coating said at least aportion of said second surface of said transparent medium so as todistinguish said at least two fiducial marks formed thereon.
 12. Themethod recited in claim 1 wherein said step of altering includes thestep of roughening said at least a portion of said second surface ofsaid transparent medium so as to distinguish said at least two fiducialmarks formed thereon.
 13. The method recited in claim 1 wherein saidstep of altering includes the step of spin coating said at least aportion of said second surface of said transparent medium so as todistinguish said at least two fiducial marks formed thereon.
 14. Themethod recited in claim 1 wherein said step of altering includes thestep of dip coating said at least a portion of said second surface ofsaid transparent medium so as to distinguish said at least two fiducialmarks formed thereon.
 15. The method recited in claim 1 wherein saidstep of forming said microlens array on said first surface and formingsaid first and second optical features on said first surface includesthe step of forming said microlens array and said first and secondoptical features by diamond milling.
 16. The method recited in claim 1wherein said step of forming said microlens array on said first surfaceand forming said first and second optical features on said first surfaceincludes the step of forming said microlens array and said first andsecond optical features by diamond turning.
 17. The method recited inclaim 1 wherein said step of forming said microlens array on said firstsurface and forming said first and second optical features on said firstsurface includes the step of forming said microlens array and said firstand second optical features by lithographic printing.
 16. The methodrecited in claim 1 wherein said step of forming said microlens array onsaid first surface and forming said first and second optical features onsaid first surface includes the step of forming said microlens array andsaid first and second optical features by ink jet printing.